An estuary is an area where a freshwater river or stream meets the ocean. In estuaries, the salty ocean mixes with a freshwater river, resulting in brackish water. Brackish water is somewhat salty, but not as salty as the ocean.

An estuary may also be called a bay, lagoon, sound, or slough.

Water continually circulates into and out of an estuary. Tides create the largest flow of saltwater, while river mouths create the largest flow of freshwater.

When dense, salty seawater flows into an estuary, it has an estuarine current. High tides can create estuarine currents. Saltwater is heavier than freshwater, so estuarine currents sink and move near the bottom of the estuary.

When less-dense freshwater from a river flows into the estuary, it has an anti-estuarine current. Anti-estuarine currents are strongest near the surface of the water. Heated by the sun, anti-estuarine currents are much warmer than estuarine currents.

In estuaries, water level and salinity rise and fall with the tides. These features also rise and fall with the seasons. During the rainy season, rivers may flood the estuary with freshwater. During the dry season, the outflow from rivers may slow to a trickle. The estuary shrinks, and becomes much more saline.

During a storm season, storm surges and other ocean waves may flood the estuary with saltwater. Most estuaries, however, are protected from the ocean's full force. Geographical features such as reefs, islands, mud, and sand act as barriers from ocean waves and wind.

Types of Estuaries

There are four different kinds of estuaries, each created a different way: 1) coastal plain estuaries; 2) tectonic estuaries; 3) bar-built estuaries; and 4) fjord estuaries.

Coastal plain estuaries (1) are created when sea levels rise and fill in an existing river valley. The Chesapeake Bay, on the East Coast of the United States, is a coastal plain estuary.

Chesapeake Bay was formed at the end of the last ice age. Massive glaciers retreated, leaving a carved-out landscape behind. The Atlantic Ocean rushed to fill in the wide coastal plain around the Susquehanna River, creating a large estuary known as a ria: a drowned river mouth.

Tectonic activity, the shifting together and rifting apart of the Earth's crust, creates tectonic estuaries (2). California's San Francisco Bay is a tectonic estuary.

The San Francisco Bay lies at the junction of the San Andreas fault and the Hayward fault. The complex tectonic activity in the area has created earthquakes for thousands of years. The San Andreas fault is on the coastal side of the bay, where it meets the Pacific Ocean at a strait known as the Golden Gate. The Hayward fault lies on the East Bay, near where the Sacramento and San Joaquin Rivers enter the estuary. The interaction of the San Andreas and Hayward faults contributes to downwarping, the process of an area of the Earth sinking.

Like the Chesapeake, the San Francisco Bay was only filled with water during the last ice age. As glaciers retreated, land around the bay experienced post-glacial rebound—without the massive weight of the glacier on top of it, the land gained elevation. The Pacific Ocean rushed in through the Golden Gate to flood the downwarped valley.

When a lagoon or bay is protected from the ocean by a sandbar or barrier island, it is called a bar-built estuary (3). The Outer Banks, a series of narrow barrier islands in North Carolina and Virginia, create sandy, bar-built estuaries.

The Outer Banks protect the region's coast from waves and wind brought by Atlantic Ocean hurricanes. The islands and sandbars also protect the delicate, brackish ecosystems created by the outflow of many rivers, such as the Roanoke and Pamlico. For these reasons, engineers monitor the shifting sandbars of the Outer Banks, and constantly work to maintain them.

Fjord estuaries (4) are a type of estuary created by glaciers. Fjord estuaries occur when glaciers carve out a deep, steep valley. Glaciers retreat and the ocean rushes into fill the narrow, deep depression. Puget Sound is a series of fjord estuaries in the U.S. state of Washington.

Like fjords found in Alaska and Scandinavia, the fjord estuaries of Puget Sound are very deep, very cold, and very narrow. Unlike many of those fjords, Puget Sound's fjord estuaries also have inflows from local rivers and streams. Many of these streams are seasonal, and fjord estuaries remain mostly salty.

Freshwater Estuaries

Some estuaries not located near oceans. These freshwater estuaries are created when a river flows into a freshwater lake.

Although freshwater estuaries are not brackish, the chemical composition of lake and river water is distinct. River water is warmer and less dense than lake water. The mixing of the two freshwater systems contributes to lake turnover—the mixing of the waters of a lake.

Freshwater estuaries are not affected by tides, but large bodies of water do experience predictable standing waves called seiches. Seiches, sometimes nicknamed sloshes, rhythmically move back and forth across a lake.

The Great Lakes, in the United States and Canada, experience seiches and have many freshwater estuaries. Old Woman Creek National Estuarine Research Center, in Huron, Ohio, was established to study the habitat created by a natural freshwater estuary. At the research center, Old Woman Creek empties into Lake Erie.

Estuary Ecosystems

Many plant and animal species thrive in estuaries. The calm waters provide a safe area for small fish, shellfish, migrating birds and shore animals. The waters are rich in nutrients such as plankton and bacteria. Decomposing plant matter, called detritus, provides food for many species.

The estuarine crocodile, for example, is an apex predator of tropical Australian and Southeast Asian estuaries. The estuarine crocodile is the largest reptile in the world. A specimen caught in the Philippines in 2011 measured 6.4 meters (21 feet).

Like most apex predators, estuarine crocodiles eat almost anything. This means the estuary must support a wide variety of food webs. Estuarine crocodiles do not usually consume producers—sea grasses, seaweeds, mushrooms, and plankton in the estuary. However, they do prey on consumers in the second trophic level, which rely on these plants and other photosynthetic organisms for food: insects, mollusks, birds, and fruit bats. Estuarine crocodiles also prey on consumers at the third trophic level, such as boars and snakes (and, rarely, people).

Estuarine crocodiles are ideally adapted to the brackish water of river estuaries. They can survive equally well in freshwater and saltwater ecosystems. During the rainy season, estuarine crocodiles live in freshwater rivers and streams. They feed on fish such as barramundi, and terrestrial species such as kangaroos and monkeys. During the dry season, estuarine crocodiles swim to river mouths and even out to sea. Fish remain the main component of their diet. Some estuarine crocodiles have even been known to attack and consume sharks.

Estuarine crocodiles have also adapted to seasonally vanishing estuaries. The reptiles can go months without eating. Estuarine crocodiles can simply not eat when the estuary shrinks and food becomes scarce.

Estuaries and People

Estuaries are excellent sites for community living. They provide freshwater for drinking and hygiene. Access to both rivers and oceans helps the development of trade and communication.

In fact, the earliest civilizations in the world developed around estuaries. Ur, in what is now Iraq, developed around 3800 BCE near the estuary of the Euphrates River where it met the Persian Gulf.

Ur was a sophisticated urban area, with a population of more than 60,000 at its height. Its estuary was the most important port on the Persian Gulf. All ships carrying trade goods from places such as India and the Arabian Peninsula had to pass through Ur. The estuary's wetlands and flood plains provided a rich source of wild game and allowed for the development of irrigation and agriculture.

Today, Ur is an archaeological site well inland from the Persian Gulf coast. The landscape has changed, and the estuary of the Euphrates is more than 300 kilometers (186 miles) away.

Many modern cities have grown around estuaries, including Jakarta, Indonesia, New York City, New York; and Tokyo, Japan. These urban areas have undergone rapid change, and put their estuaries at environmental risk through land reclamation, pollution, and overfishing.

Land Reclamation
Communities have filled in the edges of estuaries for housing and industry since the times of Ur. This process is called land reclamation.

Jakarta's 10 million residents have one of the highest population densities in the world. To create more space for homes and businesses, Indonesian officials have dredged the Ciliwung River and Java Bay. The sand and silt dredged from the river bottom and seafloor fortify the city's beaches and create new land.

Land reclamation comes at a price, however. Jakarta's fisheries are disrupted by the dredging. This reduces the potential profits for restaurants and markets, as well as fishers.

Destroying the estuary also creates the conditions for flooding. Estuaries provide a natural barrier to ocean waves, which can erode the shoreline and destroy coastal homes and businesses. Jakarta is particularly at risk for tsunami damage, as the area experiences frequent earthquakes.

Pollution
Pollution accumulates in estuaries. The Hudson-Raritan Estuary, where the Hudson and Raritan rivers meet the Atlantic Ocean, is one of the most-trafficked and most-polluted estuaries in the world.

Pollution from ships routinely spills into the Hudson-Raritan Estuary, just south of New York City. Debris in the estuary, including fuel, garbage, sewage, and ballast, remained unregulated for decades.

Runoff from agriculture and industry in New York and New Jersey also contributed a toxic estuarine environment. Industrial waste and pesticides travel downstream and settle in the water and sediment of the estuary.

Today, strict regulations and community activities are working to protect and restore the Hudson-Raritan Estuary. The restoration of oyster beds is an important part of many projects.

Oysters are a keystone species in the estuary, filter feeders that naturally help regulate toxins in the water. Millions of oyster beds greeted Henry Hudson when he entered the river in 1609. By the middle of the 20th century, however, the few remaining oysters were too toxic for human consumption. Today, several environmental groups are establishing oyster beds to repopulate the region's native species and reduce pollution in the estuary.

Overfishing
Many estuaries have been overfished. Pacific bluefin tuna are not endangered, but their range has been drastically reduced. Japan provides one of the largest markets for bluefin tuna, and the fish used to swim in the estuary of Tokyo Bay.

Bluefin tuna are large, predatory fish. They require an expansive habitat and many kilograms of food every day. As Tokyo's population grew and technology made it easier to catch more fish with less time and money, Tokyo Bay's bluefin tuna population shrank.

Today, there is not a bluefin tuna population in Tokyo Bay. However, Japanese scientists have established a successful tuna fish farming technique. Farm-raised tuna does not have a direct environmental impact on the Tokyo Bay estuary.

Indonesian, American, and Japanese governments and environmental groups struggle to promote sustainable development in estuaries. Sustainable development aims to preserve the environment while satisfying people's economic standard of living.